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The adsorption ability of natural bentonite of Slovak provenience was compared with that of composite material based on bentonite. The magnetic composite material was prepared using the method of precipitation of magnetic nanoparticles (Matik, 2004) on the surface of a selected support – bentonite, at temperatures 20 and 85 °C in different weight ratios of bentonite / magnetic particles. The surface and pore properties of natural bentonite as well as the magnetic composite were studied using the low nitrogen adsorption method. It has followed from the adsorption study that the clays under study are mesoporous. The increased values of the specific surface area and total pore volume of the composite material should be attributed to magnetic nanoparticles forming a secondary mesoporous structure. The natural and the modified bentonite have been used in sorption experiments in which the adsorption of toxic metals (zinc, cadmium and nickel) from model solutions were studied. The sorption experiments, evaluated by the Freundlich and the linearized Langmuir sorption model, revealed that the use of magnetic bentonite was more favourable. In case of a lower initial concentration of toxic metals in model solutions, the efficiency of sorption was more than 90%. The magnetic modification of natural bentonite seems to be an interesting way to enhance its sorption ability.
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ISSN 1392 – 1231. CHEMINĖ TECHNOLOGIJA. 2009. Nr. 1 (50)
47
Adsorption properties of modified bentonite clay
A. Mockovčiaková, Z. Orolínová
Institute of Geotechnics, Slovak Academy of Sciences,
45 Watsonova STR., 043 53, Košice, Slovak Republic
E-mail: mocka@saske.sk
Received 2 February 2009; Accepted 17 February 2009
The adsorption ability of natural bentonite of Slovak provenience was compared with that of composite material based on
bentonite. The magnetic composite material was prepared using the method of precipitation of magnetic nanoparticles (Matik,
2004) on the surface of a selected support – bentonite, at temperatures 20 and 85 °C in different weight ratios of bentonite /
magnetic particles. The surface and pore properties of natural bentonite as well as the magnetic composite were studied using
the low nitrogen adsorption method. It has followed from the adsorption study that the clays under study are mesoporous. The
increased values of the specific surface area and total pore volume of the composite material should be attributed to magnetic
nanoparticles forming a secondary mesoporous structure. The natural and the modified bentonite have been used in sorption
experiments in which the adsorption of toxic metals (zinc, cadmium and nickel) from model solutions were studied. The
sorption experiments, evaluated by the Freundlich and the linearized Langmuir sorption model, revealed that the use of
magnetic bentonite was more favourable. In case of a lower initial concentration of toxic metals in model solutions, the
efficiency of sorption was more than 90%. The magnetic modification of natural bentonite seems to be an interesting way to
enhance its sorption ability.
Introduction
Clays are hydrous aluminosilicates composed of
mixtures of fines-grained clay minerals, crystals of other
minerals and metal oxides. Clays play an important role
in the environment by taking up cations and anions
through adsorption or ion exchange. They belong to low-
cost sorbents [1], as the cost is an important parameter.
The good adsorption ability comes from their negative
charge which can be neutralised by adsorption of
positively charged anions. Clays obtaining montmoril-
lonite are referred to bentonite which belongs to the 2 : 1
clay family composed of two tetrahedrally coordinated
sheets of silicon surrounding and an octahedrally co-
ordinated sheet of aluminium ions. Clay can be modified
to improve its sorption ability [2, 3]; one of the methods
is the use of magnetic modification in which, e.g.,
bentonite coated with iron can be obtained [4]. The term
‘iron oxide’ is often used in scientific literature to
describe the group of iron compounds with hydroxide,
oxyhydroxide and oxide structures. They are widely used
in practice as pigments, catalysts, sorbents, in ferrofluids
[5], etc., and the route of chemical synthesis has a
significant influence on their chemical, structural and
physical properties.
The aim of this work was to study the sorption
properties of magnetic composites while removing heavy
metals (Zn, Cd and Ni) from model solutions and to
compare their adsorption capacity with that of un-
modified bentonite.
Methods
Magnetic bentonite was prepared from a solution
containing a mixture of Fe(II) and Fe(III) salts and
bentonite, treated by the sedimentation method. Two
different bentonite/iron oxide weight ratios – 1 : 1, 5 : 1 –
denoted as A and E were studied. Magnetic particles were
precipitated dropping a solution of NH4OH during a
continuous stirring for 30 min in a nitrogen atmosphere at
two ambient temperatures and 85 °C. The final products
were washed with de-ionized water to remove unfixed
iron oxide and dried at a temperature of 70 °C [6].
For characterization of the surface and pore pro-
perties of the prepared composites, nitrogen adsorption
experiments were carried out at 77 K with an ASAP 2400
sorption apparatus (Micrometrics). The specific surface
area SBET was calculated from the adsorption isotherms
according to the BET (Brunauer, Emmett and Teller)
method in a relative pressure range 0.03–0.2 p/p0. The
value of the total pore volume Va was determined from
the maximum adsorption at a relative pressure of 0.99
p/p0. The micropore volume Vmicro as well as the value of
external surface St were obtained from the t-plot analysis.
The pore size distribution of the studied samples was
calculated using the BJH (Barett–Joyner–Halenda)
method from the desorption isotherms.
The sorption of zinc, nickel, and cadmium cations
from model aqueous solutions by natural and modified
bentonites was carried out using batch-type equilibrium
experiments in a rotary shaker for 24 hours at a constant
ambient temperature. The measured dependence of
adsorption capacity on pH revealed that over pH 6.5 the
capacity decreased, and for the sorption processes, pH 5
was chosen. The initial metal ion concentration was
changed in the range 1–750 mg/l, and the amount of
sorbent was 2 g/l. The final metal concentration was
determined by atomic absorption spectroscopy (AAS
using a Varian Spectr AA-30) and the metal uptake was
calculated from the difference. The isotherms at zinc
sorption were fitted with the Langmuir equation.
48
Results and discussion
The adsorption / desorption isotherms of natural and
modified bentonites are shown in Figures 1, 2. Their
characteristic features, hysteresis loops are associated
with the capillary condensation in mesopores.
The arising final parts of the isotherms indicate an
occurrence of macropores in the pore structures of the
samples [7]. Table summarizes the values of the BET
surface area, external surface and total pore volume. The
higher values of the specific surface area and the
increased values of total pore volumes are related to the
agglomerated structure of iron oxide particles, which has
been created during the precipitation process. As seen
from Figure 2, the pore structure of synthesized pure iron
oxides contains mainly mesopores forming an almost
symmetrical distribution curve (Fig. 3). The calculated
micropore volume Vmicro of the samples has shown that
the contribution of small pores to the total pore volume is
not significant.
Fig. 1. Isotherms of bentonite and bentonite composite
Table. Structural parameters of natural bentonite and composites
Sample SBET,
m2/g
Va,
cm3/g
Vmikro,
cm3/g
St,
m2/g
Fractal dimension D
Bentonite 39.4 0.096 0.005 27.6 2.72
A20 73.7 0.216 0.004 64.2 2.57
E20 90.7 0.187 0.002 83.9 2.63
A85 82.8 0.251 0.004 73.7 2.52
E85 84.8 0.183 0.003 77.5 2.62
Fig. 2. Isotherms of bentonite, composite E and synthesized
iron oxides
The pore size distribution of the samples is shown in
Figure 3, where an increase in the pore volume of
composite materials is also noticeable. A comparison of
the modified bentonites A and E shows that a higher pore
volume was obtained at sample A where the content of
magnetic particles was higher. The effect of synthesis
temperature of samples A was not expressive. The poro-
sity of the composite samples E was different, depending
on the synthesis temperature: the distribution curve
corresponding to the magnetic composite prepared at an
ambient temperature is shifted to the smaller pore
diameters, implying out that the size of magnetic particles
precipitated on bentonite at the ambient temperature is
smaller than in case of the synthesis temperature 85 °C. It
can explain the value of the highest specific surface of the
composite sample E20; the highest total pore volume in
sample A85 can be attributed to the content of magnetic
particles. These two mentioned samples were selected for
the sorption experiments.
Fig. 3. Pore size distribution in the sample
49
The sorption of zinc, nickel and cadmium from
aqueous solutions was carried out on natural bentonite
and on composites A85 and E20. The amount of metal
ions Zn2+ adsorbed on natural and modified bentonite E20
as a function depending on its initial concentration in the
range 10–100 mg/l was studied. The obtained isotherms
were fitted with the linearized Langmuir type model
(Fig. 4) according to the equation [8]: synthesis
m
e
me
e
Q
C
KQq
C+=
.
1,
where Ce is the equilibrium concentration of metal in solution,
and qe denotes the amount of sorbed metal ions. The Langmuir
maximum sorption capacity Qm of bentonite and E20, calculated
from the slopes of the plots, were 22 mg Zn/g and 23 mg Zn/g,
respectively.
Fig. 4. Adsorption of Zn(II) on natural and modified bentonite
E20
Nickel sorption on natural bentonite and composite
E20, carried out in dependence on the initial metal
concentration range 10–750 mg/l, is shown in Figure 5.
The isotherms fitted by the Freundlich model show, that
in spite of wider range of the initial concentration, the
maximum sorption capacity was still not attained.
Comparing the sorption isotherms in Figure 5, an
increase in Ni sorption on the composite bentonite E20 is
observable in the range of lower concentrations. Another
sorption experiment, including the concentration range 1–
15 mg/l, was carried out (Fig. 5). It followed from the
experiment that the magnetic composite E20 showed at a
lower concentration range a very good sorption capacity;
in comparison to natural bentonite, a more than 90%
sorption efficiency of composite material was achieved.
The sorption of cadmium was investigated on three
sorbents including natural bentonite, as well as
composites A85 and E20. The effect of metal ion initial
concentration on the amount of sorbed ions was
investigated over the concentration range 1–50 mg/l at a
constant ambient temperature. The experimental data on
sorption were fitted with the linearized Langmuir type
model (Fig. 6). The calculated sorption capacities of
natural bentonite, composites A85 and E20 were 27.7 mg
Cd/g, 23.7 mg Cd/g and 29.4 mg Cd/g, respectively. It
can be seen in Figure 6 that the increase in the amount of
the sorbent ions on composites A85 and E20 was most
expressive again in a lower concentration range. Another
sorption experiment showed that both composite sorbents
A85 and E20 were very effective in comparison with
natural bentonite at Cd2+ ions sorption in the initial
concentration range 1–10 mg/l.
Fig. 5. Sorption of nickel on bentonite and composite E20
Fig. 6. Sorption of cadmium on bentonite, composites A85 and
E20
It can be concluded that the magnetically modified
bentonite samples A85 and E20 should be more
convenient in removing metals from an aqueous solution
when their concentration is very low but still harmful.
Conclusions
The composite sample prepared in bentonite / iron
oxide weight ratio 5 : 1 at a temperature of 20 °C showed
a higher amount of Zn2+ ions sorbed over the whole
concentration range (10–100 mg/l) in comparison to
natural bentonite. The sorption of Ni2+ ions in a lower
initial concentration range (1–15 mg/l) was very
50
favourable for the same magnetic composite sorbent. In
case of the sorption of Cd2+ ions, the use of magnetic
sorbents prepared in two bentonite / iron oxide weight
ratios, 1 : 1 and 5 : 1, at a temperature of 85 °C and 20 °C
respectively, was very convenient. The magnetically
modified bentonite seems to be a promising candidate for
the practical use in the removal of heavy metals.
Acknowledgements
The authors are grateful to the Slovak VEGA Grant
Agency for financial support of the project G/0119/29.
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A. Mockovčiaková, Z. Orolínová
MODIFIKUOTO BENTONITINIO MOLIO
ADSOBCINĖS SAVYBĖS
S a n t r a u k a
Darbe lyginamos gamtinio bentonito (Slovakija) ir kom-
pozicinių medžiagų su bentonitu adsorbcinės galimybės. Mag-
netinės kompozicinės medžiagos buvo ruošiamos pagal magne-
tinių nanodalelių nusėdimo ant pasirinkto pagrindo – bentoni-
to – metodą (Matik, 2004) 20 ir 85 oC temperatūroje, esant skir-
tingiems bentonito / magnetinių dalelių svoriniams santykiams.
Gamtinio bentonito paviršiaus ir porų savybės yra tokios pat
kaip ir magnetinio kompozito, kuris buvo tirtas, remiantis azoto
adsorbcijos metodu. Taigi, pagal adsorbcijos tyrimus molis yra
mezoporė medžiaga. Kompozicinės medžiagos didesnis savitojo
paviršiaus plotas ir suminis porų tūris turėtų priklausyti nuo
magnetinių nanodalelių, susidarant mezoporei struktūrai. Sorb-
cijos darbuose buvo naudojami gamtinis ir modifikuotas bento-
nitas ir tirta toksiškų metalų (cinko, kadmio ir nikelio) adsorb-
cija iš tirpalų. Atlikus sorbciją pagal Freundličo (Freundlich) ir
linijinį Lengmiuro (Langmuir) sorbcijos modelius, nustatyta,
kad minėtiems tikslams geriau naudoti magnetinį bentonitą. Kai
tirpaluose yra mažesnės pradinių toksiškų metalų koncentra-
cijos, sorbcijos našumas yra daugiau nei 90 %. Taigi mo-
difikavus gamtinį bentonitą, padidėja jo sorbcinės galimybės.
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Clay is one of the most important industrial minerals that have been used to improve the quality of product and economize the production cost. Clay and their minerals, both in its natural and modified forms, have the ability to absorb various radioactive materials from aqueous solution, such as Uranium, Thorium and Caesium as discussed extensively in this review. This article presented an overview of properties and classifications of clay, current research literature on using clay minerals as an absorber, and a descriptive analysis of their adsorption behaviour. Three type of clay are the focused in this review namely Bentonite, Kaolin and Zeolites due to their excellent qualification in absorbing radioactive materials such as Uranium, Thorium and Caesium.
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The paper is devoted to the utilization of ferrofluid as a separating medium and modifying agent affecting the magnetic properties of solid and liquid materials. The separation tests in a MHS separator have been carried out under laboratory condition. The selectivity of ferrofluid's adsorption to the surface of some materials can be used for enhancing the magnetic susceptibility and influencing the efficiency of separation process. The enhancement of magnetic susceptibility of oil products up to a level sufficient for their magnetic extraction from water is possible by admixing of a definite amount of kerosene-based ferrofluid, which is non-miscible with water. The results point to the fact that the MHS method is suitable for the separation of non-magnetic materials according to their density and the modification of magnetic properties of materials by ferrofluid enhancing their magnetic separability.
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